1. Field of the Invention
The invention relates in general to electrical apparatus, such as electrical transformers, electrical cables, and the like, and more specifically to protective systems and methods for detecting overheating of such apparatus.
2. Description of the Prior Art
The type of prior art smoke detector which includes an ion chamber, utilizes a radioactive source which emits alpha particles. The alpha particles ionize the air and the ions are collected via a charged collector electrode. In the absence of particulates, such as smoke, in the air, a steady state ion current flows through the collector electrode.
Smoke particles entering the ion chamber reduce the ion current, as the ions attach themselves to the smoke particles. The resulting combination is heavier than the ions themselves, reducing the number of ions which reach the collector electrode. An alarm circuit detects when the ion current drops from the steady state value to a predetermined smaller magnitude, and an alarm is sounded when the predetermined lower value is reached.
Some prior art ionization chamber type smoke detectors have been set up to recognize the addition of ionized air caused by the combustion gases of smokeless fires. These detectors detect an increase in the ion current from the steady state value, and alarm on a predetermined increase. U.S. Pat. Nos. 2,408,051 and 3,795,904 disclose examples of this type of detector.
Co-pending application Ser. No. 751,403, filed Dec. 16, 1976, which application is assigned to the same assignee as the present application, detects overheating of electrical apparatus cooled by a stream of hydrogen, such as hydrogen cooled dynamoelectric machines. A compound is provided within the apparatus which decomposes between 80 and 200.degree.C. to provide a gas heavier than hydrogen. The recombination rate of the hydrogen ions is slower in the presence of this gas, thus increasing the ion current from the steady state value when this gas is present. An ion chamber monitor detects an increase in ion current caused by the presence of this gas in the hydrogen, to sound an alarm.